Silicon wafers are widely used as a substrate material for semiconductor devices. The silicon wafers are manufactured by sequentially applying processes such as outer peripheral grinding, slicing, lapping, etching, double-side polishing, single-side polishing, washing, etc., to a silicon single crystal ingot. Among the above processes, the single-side polishing is a process required in order to remove unevenness or waviness on the wafer surface and thus to enhance flatness, in which mirror-finishing by CMP (Chemical Mechanical Polishing) method is performed.
Typically, in the single-side polishing process for a silicon wafer, a single wafer polishing apparatus (CMP apparatus) is used. The wafer polishing apparatus includes a rotating platen to which a polishing pad is affixed and a polishing head that holds a wafer on the polishing pad while pressing the wafer. The apparatus polishes one side of the wafer by rotating the rotating platen and polishing head while feeding slurry.
In order to enhance uniformity of the polishing amount in a wafer surface by uniformly pressing the entire surface of the wafer, a wafer polishing apparatus using a membrane pressurization type polishing head is used. For example, Patent Document 1 describes a wafer polishing apparatus having a membrane that transmits a pressing force to a wafer on the polishing pad and a tension adjusting means for adjusting tension of the membrane. Further, in order to make the polishing amount in a wafer surface uniform, Patent Document 2 describes a polishing head in which areas having different surface roughnesses are concentrically disposed on a wafer holding surface of a membrane (elastic body film). The areas include a first area formed from the outer periphery of the membrane to a section from 10% to 80% of the radius thereof and having an arithmetic-mean roughness Ra of 5.0 μm to 50.0 μm and a second area formed inside the first area having an arithmetic-mean roughness Ra of less than 5.0 μm. Further, Patent Document 3 describes a membrane for a polishing head having a circular bottom part, a wall part rising from the outer edge of the circular bottom part, an annular part extending from the upper end of the wall part to the central direction of a circle, and a projection part located at the terminal end of the annular part.